The present invention relates to excavating tools such as revolving cutter head excavators for use in mines or dredgers.
Revolving cutter head excavators consist of a drive wheel that rotates around a shaft and is driven by a means of rotation. The periphery of the revolving cutter head excavator has a series of buckets equipped with teeth arranged in directions that are essentially radial. Dredgers do not have buckets and their teeth are distributed around the periphery in a rotary ogival structure. Each tooth consists of a single-unit tooth body structure made of a mechanically resistant metal or alloy such as steel, having a fixing area to connect it to the bucket or the ogival structure and a working area to dig the soil. The working area is generally flat and shaped like a shovel and is bounded by a leading face that points in the direction of movement of the periphery of the wheel or ogival structure in the preferred direction of rotation and a trailing face or face opposite the leading face. The leading face and the trailing face are generally flat or slightly curved and are connected by a front tapered facet that defines a transverse cutting edge. If the tooth is mounted on the bucket or the ogival structure, the transverse cutting edge is essentially parallel to the axis of rotation of the assembly and the general plane formed by the tooth shovel or working area generally slants in the direction of the direction of movement of the tooth in the preferred direction of rotation.
During operation, part of the peripheral zone of the bucket or cutter cuts into the ground, the transverse cutting edge of the teeth bites into the ground and the leading face pushes up the material. This results in considerable wear of the transverse cutting edge and the leading face.
One common solution to increase the service life and the efficiency of the teeth is to hardface the external surface of the leading face and the tapered front facet in order to cover them with a coat of molten carbide by fusing a welding bead.
Although this process significantly increases the service life of the tooth, wear still occurs, relatively slowly at the start of use when the hard material still covers the front facet; wear then becomes much faster when the hard material that covers the front facet is itself damaged by wear. The tooth can only be used as long as the length of its working area has not reduced too extensively and this defines the maximum permissible area of wear of the tooth.
In particular, as soon as the front facet has lost its protective coating of hard material, wear becomes much faster despite the existence of a layer of hard material on the leading face of the tooth.
Another drawback of known structures is that they require the use of a protective surface hardfacing, a layer that is produced by melting with a welding torch or an electric arc. An examples of such a layer comprises a deposit consisting of a mixture of molten carbide particles embedded in a fusible matrix. Such hardfacing is time consuming and awkward and produces relatively irregular surfaces made up by the juxtaposition of several side-by-side welding beads. The intermediate areas between two successive beads are usually sunken areas of which the metallurgical structure is slightly different from the central structure of the welding beads. This results in a lack of homogeneity of the material that forms the protective layer made of a hard material and this results in the appearance of preferential areas of wear, thus encouraging faster wear of the material. In addition, such a process is expensive and requires skilled labor.
Dredger teeth with a composite structure are known consisting of a metal tooth body containing inserts of a hard anti-abrasion material. In document U.S. Pat. No. 3,805,423, a prefabricated insert is fitted in appropriate recesses in the metal tooth body where it is fixed by welding or brazing. The insert, in the embodiment shown in FIGS. 3 and 4, consists of two intermediate bars which each take up half the height of the tooth. Document U.S. Pat. No. 4,052,802 also describes providing a prefabricated insert and fitting it in the tooth body. The insert is sandwiched between the metal surface plates, between which it is assembled by brazing. Therefore the insert does not take up the entire height of the tooth. There is no suggestion in this document of replacing the metal plates by a material containing particles of a hard material.
In document FR-A-2 373 500, an excavating tool is produced by providing cover plates made of sintered carbide on a steel body. The steel body is cast around the cover plates.
There is no suggestion in this document of replacing the internal steel body by a material containing particles of hard material. In any case, this results in an extremely fragile tooth.
The structures and production processes described in documents U.S. Pat. No. 4,052,802 and FR-A-2 373 500 are not compatible with each other. In fact, producing a tooth with an internal insert made of particles of hard material in accordance with document U.S. Pat. No. 4,052,802 is achieved by assembling several subassemblies by brazing whereas document FR-A-2 373 500 makes provision for such assembly by molding from a casting. The expert is therefore not encouraged to cosine the teachings of instruction in these two documents.
In document U.S. Pat. No. 3,286,379, fingers of hard material are produced by casting a hard material in longitudinal grooves in the metal tooth body. Document JP-A-62 99 527 describes a tooth for an excavating tool in which the prefabricated inserts are formed from sintered carbide and are assembled on the tooth body by brazing.
It seems that these known structures with longitudinal inserts do not give the expected results in terms of efficiency and long service life. In fact, fairly rapid wear is observed on the tooth, particularly due to flaking of the bars made of hard material. The bars of hard material which do not take up the entire height of the tooth do not provide a sufficient increase in the service life of the tooth and their manufacturing process does not allow sufficient cohesion of the components of such a heterogenous structure.
One of the main objects of the present invention is to avoid the disadvantages of known excavating tool teeth structures and their production processes; it initially proposes a new composite tooth structure consisting of several longitudinal bars of hard material that take up the entire height of the tooth. The new tooth structure is compatible with the presence of protective surface layers made of a molten hard material but can also be used without such a protective surface layer.
One of the problems is that, with usual brazing or welding processes, it is awkward or difficult to correctly insert and join bars that take up the entire height of the tooth without adversely affecting the mechanical properties of the anti-abrasion material that constitutes the bars. The invention solves this difficulty by using a new infiltration process on the tooth blank itself.
The invention suggests producing such a tooth structure by means of a so-called infiltration process. The infiltration process can be implemented in a relatively simple manner and does not require great skill on the part of the user, unlike hardfacing techniques using a welding bead, and also results in lower production costs. The process avoids the tricky operation of having to solder or braze an insert.
When such an infiltration process is used, the tooth structure thus obtained is characterized by the fact that the bars of hard material are bonded to the metal of the tooth body by a brazing alloy that forms the matrix which itself links the particles of hard material to each other. This feature seems particularly important in order to obtain satisfactory cohesion between the bars of hard material and the metal that forms the tooth body.
When using such an infiltration process, the mold structures are particularly small and easy to produce because the metal parts of the tooth structure themselves act as a mold.
The invention makes it possible to considerably improve the service life and efficiency of an excavating tool tooth to a surprising extent compared with familiar techniques given comparable quantities of hard material. The tooth continues to cut as it wears.
Finally, the risk of breakage or flaking of the coating and the bars of hard material is significantly reduced; this risk is often encountered with known teeth.
The invention therefore makes it possible to obtain better cohesion of the excavating tooth, improved hardness of the bars of hard material and greater ease of production.